No they aren’t the same thing. Backhanded compliment = your makeup looks great,it covers up your bad skin flawlessly. Sarcasm= [you are visibly struggling while carrying a heavy item] friend: do you need help? You: no, of course not, I enjoy risking breaking my back.

Sarcasm isn't a complement it has a deeper meaning. Back handed complement is an insult coated as complement. Nothing deeper

Backhanded compliment is what you say all the time :)

Well no. They're not the same really. A backhanded compliment would be you throwing out a "compliment" that has an insult or underlying negative tone attached to it. Sarcasm is very much used as a means to taunt/tease people, generally in a more humorous way, not always but mostly. Often it its very context dependent and a response to a rather obvious thing.